The reversed apical pattern of MUC1 expression is characteristics of invasive micropapillary carcinoma of the breast.

BACKGROUND: Invasive micropapillary carcinoma (IMPC) of the breast is a special subtype of invasive ductal carcinoma (IDC), which is known to have a high potential to metastasize to the axillary lymph node. However, it is sometimes difficult to differentiate IMPC from conventional IDC showing an IMPC-like pattern due to artifact (pseudo-IMPC). In the present study, we investigated the usefulness of immunohistochemical expression of MUC1 for distinguishing IMPC from pseudo-IMPC, and analyzed several clinicopathological parameters of IMPC and pseudo-IMPC cases.

METHODS: Eighty cases showing IMPC or IMPC-like pattern were selected from our surgical files of 1,240 cases of IDC. We examined the expression of MUC1, D2-40 and CD34 by immunohistochemistry.

RESULTS: Eighty cases were classified into 9 cases (0.7%) of pure-IMPC, 31 cases (2.5%) of mixed-IMPC, and 40 cases of pseudo-IMPC, according to the expression pattern of MUC1. In pure-IMPC cases, MUC1 expression was found at the reversed apical membrane of neoplastic cell clusters, while in pseudo-IMPC, MUC1 expression was present in the whole cytoplasmic membrane and/or cytoplasm. There were no significant differences among the three groups in patient age, tumor size and nuclear grade of neoplastic cells. However, lymphatic invasion and lymph node metastasis in the pure-IMPC or mixed-IMPC cases were higher than those in pseudo-IMPC cases with statistically significant values. Pure-IMPC has a higher recurrence rate and lower overall survival compared to pseudo-IMPC [P = 0.0165(DFS) P = 0.025(OS)].

CONCLUSIONS: This study demonstrated that immunohistochemistry of MUC1 is useful for the diagnosis of IMPC. The pure-IMPC cases had higher incidences of lymphatic invasion and lymph node metastasis, and also showed a poorer prognosis.